1. Field of the Invention
The present invention relates to a digital control circuit that performs bidirectional communication between a primary side and a secondary side, driving of a switching element with a low loss, and acquisition of power from a path for the bidirectional communication, and a switching power supply apparatus including the digital control circuit.
2. Description of the Related Art
In isolated switching power supply apparatuses, in order to use different ground potentials in a primary circuit and a secondary circuit and satisfy a safety standard, a primary side and a secondary side are isolated using a transformer. In order to control an output voltage and an output current of the secondary side, the output voltage and the output current of the secondary side are detected and are then fed back to the primary side and a switching circuit on the primary side is controlled on the basis of the detected output voltage and the detected output current. At that time, a feedback circuit also needs to achieve the isolation between the primary side and the secondary side.
When a synchronous rectification circuit is used on the secondary side, in order to prevent a case in which a main switch and a commutator-side synchronous rectifier are turned on at the same time and a flow-through current flows, it is necessary to turn off the commutator-side synchronous rectifier in a secondary-side synchronous rectification circuit immediately before the main switch is turned on. Accordingly, a signal indicating a time immediately before the main switch is turned on must be transmitted from a primary side to the secondary side so as to control a time of switching performed by the secondary-side synchronous rectification circuit (see Japanese Unexamined Patent Application Publication No. 2002-272097). FIG. 1 illustrates a basic configuration of an isolated DC-DC converter disclosed in Japanese Unexamined Patent Application Publication No. 2002-272097.
Referring to FIG. 1, a main switch 2 is subjected to switching control in accordance with a control signal output from a primary-side control unit 5. The primary-side control unit 5 detects an output voltage Vo via an isolation circuit 10 and outputs a control signal C1 for controlling a duty ratio of the main switch 2 on the basis of the output voltage Vo. The control signal C1 is also transmitted to a secondary side via driving circuits 7 and 8 and a transformer 9 and becomes a control signal C2. The control signal C2 is supplied to a secondary-side control unit 21. The control signal C2 is transmitted from the secondary-side control unit 21 to an input terminal of a driving circuit 13 and a gate electrode of a transistor 15. As a result, a rectifier-side synchronous rectifier 3 is driven in phase with the main switch 2, and a commutator-side synchronous rectifier 4 is driven in opposite phase with the main switch 2.
A time difference between the control signals C1 and C2 that is caused by the driving circuits 7 and 8 and the transformer 9 and a delay time of turning off of an MOSFET in the commutator-side synchronous rectifier are adjusted by a delay circuit 11 for delaying turning on of the main switch.
As illustrated in FIG. 1, when a synchronous rectification circuit is used on the secondary side, at least two units including a unit for feeding back an output voltage detection signal to the primary side are needed to transmit a signal between the primary side and the secondary side while insulating the primary side and the secondary side from each other. As a result, a circuit configuration therefore becomes complicated.
When the main switch and the synchronous rectifier on the secondary side are MOSFETs, an electric charge stored in an input capacitor is discharged every switching period. Accordingly, the larger the die size of a MOSFET and the higher a switching frequency, the larger a driving loss.
In recent years, ICs including a digital control circuit have been used. In order to achieve a low-loss digital control circuit, a voltage for driving the digital control circuit is generally lower than a voltage for driving a main switch. For example, a driving voltage for a main switch is approximately 10 V, and a driving voltage for a digital control IC is approximately 2 V. When a linear regulator or the like is used to obtain two driving voltages from a single power supply voltage, the conduction loss of the linear regulator becomes pronounced. When a switching regulator for supplying a driving voltage to a digital control circuit is used, a circuit configuration becomes complicated.